The present invention relates to magnetic resonance and, more particularly, to a method, system and radiofrequency pulse sequence for magnetic resonance analysis and/or imaging of a body having therein water molecules and one or more molecular species capable of magnetically interacting with the water molecules.
Magnetic resonance imaging (MRI) is a method to obtain an image representing the chemical and physical microscopic properties of materials, by utilizing a quantum mechanical phenomenon, named Nuclear Magnetic Resonance (NMR), in which a system of spins, placed in a magnetic field resonantly absorb energy, when applied with a certain frequency.
A nucleus can experience NMR only if its nuclear spin I does not vanish, i.e., the nucleus has at least one unpaired nucleon. Examples of non-zero spin nuclei frequently used in MRI include 1H (I=1/2), 2H (I=1), 23Na (I=3/2), etc. When placed in a magnetic field, a nucleus having a spin I is allowed to be in a discrete set of energy levels, the number of which is determined by I, and the separation of which is determined by the gyromagnetic ratio of the nucleus and by the magnetic field. Under the influence of a small perturbation, manifested as a radiofrequency magnetic field, which rotates about the direction of a primary static magnetic field, the nucleus has a time dependent probability to experience a transition from one energy level to another. With a specific frequency of the rotating magnetic field, the transition probability may reach the value of unity. Hence at certain times, a transition is forced on the nucleus, even though the rotating magnetic field may be of small magnitude relative to the static magnetic field. For an ensemble of spin I nuclei the transitions are realized through a change in the overall magnetization.
Once a change in the magnetization occurs, a system of spins tends to restore its magnetization longitudinal equilibrium value, by the thermodynamic principle of minimal energy. The time constant which control the elapsed time for the system to return to the equilibrium value is called “spin-lattice relaxation time” or “longitudinal relaxation time” and is commonly denoted T1. An additional time constant, T2 (≦T1), called “spin-spin relaxation time” or “transverse relaxation time”, controls the elapsed time in which the transverse magnetization diminishes, by the principle of maximal entropy. However, inter-molecule interactions and local variations in the value of the static magnetic field alter the value of T2, to an actual value commonly denoted T2*.
In MRI, a static magnetic field having a predetermined gradient is applied on an object, thereby creating, at each region of the object, a unique magnetic field. By detecting the NMR signal, knowing the magnetic field gradient, the position of each region of the object can be imaged.
In MRI, pulse sequences are applied to the object (e.g., a human or animal) to generate NMR signals and obtain information therefrom which is subsequently used to reconstruct images of the object. The produced image is affected by parameters such as spin density, transverse and longitudinal relaxation times, residual dipolar interactions in anisotropic media (such as fibrous biological tissues), chemical exchange between proteins, membranes and water. The aforementioned relaxation times and the density distribution of the nuclear spin are properties which vary from one normal tissue to the other and from one diseased tissue to the other. These quantities are therefore responsible for contrast between tissues in various imaging techniques, hence permitting image segmentation.
A common characteristic for many MRI techniques is that the properties of water molecules are measured, which properties are indirectly dependent on interaction with macromolecules such as proteins.
Over the years, many MRI method have been developed to meet the requirements of contrast enhancement. Representative examples of such methods include T1 weighted MRI, T2 weighted MRI, fat suppression MRI and diffusion weighted MRI.
Of particular relevance to the present invention are methods in which the contrast between tissues is increased by non-invasive physical means. One such method is known as the Magnetization Transfer Contrast (MTC) method [S. D. Wolff and R. S. Balaban, Magn. Reson. Med., 10, 135 (1989)]. For this technique to be effective, there must be at least two spin systems in the imaged anatomy which are capable of exchanging energy between themselves and should be distinguished from one another by a certain magnetic resonance parameter. One such distinguishing parameter is the transverse relaxation time. A typical example for such two spin systems is protein, with a short T2, and water with a long T2.
Due to the inverse relationship between T2 and the NMR spectral linewidth, a broad peak would be observed from the protein and a narrow peak would be observed from the water, had the two systems been measured separately. However, when these systems are imaged, the acquisition of the signal from the protein is practically impossible because for most clinical systems this signal decays off prior or very shortly after such acquisition starts. According to the MTC method, the above difficulty is resolved, by applying an appropriate pulse sequence that saturates the protein spin system and not the water. Consequently, the exchange of magnetization between water molecules and macromolecules can be detected. Hence, saturating the protein ensures contrast between water being in contact with the protein and water being far from the protein.
Although MTC is a method which is dependent on the nature of the proteins, it has two major drawbacks. First, in MTC signals indicating magnetization transfer from the protein to the water are entangled with signals indicating direct excitation of the water molecules. Secondly, the MTC is not directly related to the amount and properties of the other species.
Also known, is a method known as Double Quantum Filter Magnetization Transfer (DQF-MT) useful for semi-rigid tissues, such as articular cartilage and tendons [International Patent Application No. WO02/052283; A. Neufeld, U. Eliav and G. Navon, “New MRI method with contrast based on the macromolecular characteristics of tissues,” Magn. Reson. Med. 50, 229, 2003]. This method is based on the residual intramolecular dipolar interaction in macromolecules and water molecules. Using a double quantum filtered (DQF) sequence, macromolecules are selectively excited. Subsequently, the magnetization is transferred to the water, allowing imaging of the water magnetization originating from the macromolecules and thus giving an image relating to the macromolecular content. The contrast in DQF MRI stems from the fact that only water molecules associated with ordered structures are detected while signals originating from molecules in isotropic tissues are suppressed.
An additional method for increasing contrast is based on the subtraction of two signals [M. Goldman and L. Shen, Spin-Spin Relaxation in Laf3, Phys. Rev. 144, 321, 1966; J. P. Renou, J. Alizon, M. Dohri, H. Robert, J. Biochem. and Biophys. Meth., 7, 91, 1983; D. Carasso, U. Eliav and G. Navon, “NMR parameters for monitoring coagulation of liver tissue,” Magn. Reson. Med. 54, 1082, 2005; U.S. Patent Application No. 60/607,589]. The signal of water obtained with one pulse sequence is subtracted from the water signal obtained with another pulse sequence. The two pulse sequences are selected such that in one pulse sequence, the magnetization transfer takes place, while in the other pulse sequence includes time-intervals which are too short for magnetization transfer.
A major disadvantage of the techniques disclosed in Neufeld et al. and Carasso et al. supra is that they require very short radiofrequency pulses having homogenous radiofrequency magnetic field. Typically, radiofrequency pulses of less than 100 μs should be generated to achieve the desired effect. However, presently available clinical MRI scanners, particularly whole body scanners in which large radiofrequency coils are deployed, require much longer radiofrequency pulses.
There is thus a widely recognized need for, and it would be highly advantageous to have, a MRI method and system for selective excitation devoid of the above limitations.